Rooster: Destroy the Behemoth logo building while out of the tank in level 1-4, then successfully complete the level.

Stove Pipe: Successfully complete level 1-1 with gore off.

Tiara: Successfully complete level 2-2 on the Hard difficulty.

Yellow Shark Fin: Dig or suffocate before riding the snowmobile in level 2-4, then successfully complete the level.

Easy Survivalist and Killing Spree achievements

Select the Area 51 Challenge. Start by killing everything in sight. Wait for the big metal robot to appear from the right. Do not destroy it; it does not kill you. As soon as it enters the screen, stand directly under it and start shooting in the opposite direction. You will easily kill everything that appears from the left side and the enemies jumping over the robot. Keep doing this for about twenty minutes to unlock both achievements at the same time.

Achievements

Accomplish the indicated achievement to get the corresponding number of Gamerscore points:

Alien in Training (5 points): Complete level 1-1.

Hungry Yeti (10 points): Eat 50 KGB Agents with the Yeti.

City Destroyer (10 points): Destroy 25 buildings during one game.

Juvenile Alien (10 points): Complete the game on the Easy difficulty.

Adult Alien (15 points): Complete the game on the Medium difficulty.

Expert Alien (20 points): Complete the game on the Hard difficulty.

Head Chomper (20 points): Head bite 50 enemies without touching the ground or firing.

Killing Spree (15 points): Kill 1,000 enemies during one game.

Mad Hatter (20 points): Unlock all 31 hats.

Perfect Flight (30 points): Defeat the final Boss without using a continue.